Stimulation of B cells with IL5 induces the appearance of a phenotypically novel B cell population which expresses high density of CD44 and low densities of B220 (CD45) and Ia. CD44 expressed by these cells mediates binding to the extracellular matrix material hyaluronic acid (HA), indicating a potential role for CD44 in trafficking of activated B cells in vivo. CD44 expressed on IL5-stimulated B cells migrates with a lower molecular weight than CD44 expressed by control B cells, reflecting differential glycosylation. No differences in CD44 mRNA isoform were apparant by PCR analysis. The B cell stimuli LPS and anti-k do not induce CD44-dependent HA-binding activity. However, LPS-activated B cells demonstrate CD44-dependent HA binding rapidly after exposure to a unique CD44-specific mAb, suggesting that distinct functional states of the CD44 molecule exist reflecting differences in conformation or cytoskeletal association. A PCR-based approach has characterized tissue-specific expression of multiple CD44 isoforms resulting from alterntive splicing of up to 10 exons. mAb were generated by immunizing rats with activated mouse B cells. One of these mAb (GL7) reacts with a subpopulation of activated B cells, as well as with activated T cells. GL7 precipitates a previously undescribed 29-31 KDa molecule from activated B cells. Another mAb (GL1) reacts with activated B cells. GL1 inhibits costimulus-dependent responses of CD4+ T cells and identifies an alternative ligand for the T cell molecule CTLA4. The molecule identified by GL1 has been shown to be B7-2, the product of a gene related to but distinct from that encoding B7 (now B7-1). B7-2 product has been shown to play a predominant functional costimulatory role both in vivo and in vitro.